Answer
See the answer below.
Work Step by Step
$pH=-log([H_3O^+])$
$[H_3O^+]=10^{-pH}$
$[H_3O^+]=0.005\ M=[F^-]$
$K_a=[H_3O^+][F^-]/[HF]$
$7.2\cdot10^{-4}=0.005^2/(x-0.005)$
$x=0.0397\ M=[HF]_0$
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